[index.html] Events are from UTC times, not PST

Looking at e.g. http://data.githubarchive.org/2012-03-08-0.json.gz

All events within that dataset have timestamps from 
"2012-03-08 00:00:00 UTC" through
"2012-03-08 00:59:59 UTC"
[index.html] UTC times, not PST - keep formatting

Don't destroy the visual hierarchy 
just to have clarity. 
[index.html] UTC times, not PST (cont.) 3PM UTC
Merge pull request #26 from tsnow/UTC-vs-PST

[index.html] Events are from UTC times, not PST